The present invention relates to a current sensor apparatus used for non-contact measurement of a relatively large electric current.
Many types of magnetic sensor apparatuses and non-contact-type electric current sensor apparatuses utilizing magnetic sensor apparatuses have been long developed since such apparatuses are useful in industry. However, their application fields have been limited and the market scale have been thus limited. Consequently, development of such apparatuses in terms of cost reduction have not been fully achieved yet.
However, emission control originating from the need for solving environmental problems has accelerated development of electric automobiles and solar-electric power generation. Since a direct current of several kilowatts to tens of kilowatts is dealt with in an electric car or solar-electric power generation, a non-contact current sensor apparatus is required for measuring a direct current of tens to hundreds of amperes. The demand for such current sensor apparatuses is extremely high. It is therefore difficult to increase the popularity of electric automobiles and solar-electric power generation unless the current sensor apparatuses not only exhibit excellent properties but also are extremely low-priced. In addition, reliability is required for a period of time as long as 10 years or more for a current sensor apparatus used in a harsh environment as in an electric car. As thus described, it has been requested in society to provide current sensor apparatuses that are inexpensive and have excellent properties and long-term reliability.
For non-contact measurement of an electric current, an alternating current component is easily measured through the use of the principle of a transformer. However, it is impossible to measure a direct current component through this method. Therefore, a method is taken to measure a magnetic field generated by a current through a magnetic sensor for measuring a direct current component. A Hall element is widely used for such a magnetic sensor. A magnetoresistive element and a fluxgate element are used in some applications, too.
For example, the following problems have been found in the current sensor apparatus utilizing a Hall element that has been most highly developed in prior art.
(1) low sensitivity
(2) inconsistent sensitivity
(3) poor thermal characteristic
(4) offset voltage that requires troublesome handling
In addition to the above problems, a magnetoresistive element has a problem of poor linearity.
In Published Unexamined Japanese Patent Application Hei 7-218552 (1995), a technique is disclosed for increasing a current measurement range of a current sensor apparatus incorporating a Hall element by forming two gaps having different lengths at separated ends of a ring-shaped core member and placing a Hall element in each of the gaps.
An example of a current sensor apparatus incorporating a magnetoresistive element is disclosed in Unexamined Japanese Patent Application Hei 10-26639 (1998).
Some methods have been developed for solving the problems of a Hall element. One of the methods is a so-called negative feedback method, that is, to apply a reversed magnetic field proportional to an output of the element to the element so as to apply negative feedback such that the output of the element is maintained constant. Consistency in sensitivity, the thermal characteristic, and linearity are thereby improved.
When the negative feedback method is used, however, it is required to apply an inverse magnetic field as large as the field to be measured to the element. Consequently, when a current as high as hundreds of amperes is measured in applications such as an electric car or solar-electric power generation, a feedback current obtained is several amperes even if the number of turns of the coil for generating a feedback field is 100. Therefore, a current sensor apparatus embodied through this method is very large-sized and expensive.
If the magnetic sensor element has high sensitivity, it is possible that a feedback current is reduced by applying only part (such as one hundredth) of the field to be measured to the element. However, this is difficult for a Hall element with low sensitivity used as the magnetic sensor element.
A fluxgate element has been developed mainly for measurement of a small magnetic field while not many developments have been made in techniques for measuring a large current. However, with some modification a fluxgate element may be used as a magnetic detection unit of a current sensor apparatus for a large current since the fluxgate element has a simple configuration and high sensitivity.
Reference is now made to FIG. 17 to describe the operation principle of a fluxgate element having the simplest configuration. FIG. 17 is a plot for showing the relationship between an inductance of a coil wound around a magnetic core and a coil current. Since the core has a magnetic saturation property, the effective permeability of the core is reduced and the inductance of the coil is reduced if the coil current increases. Therefore, if bias magnetic field B is applied to the core by a magnet and the like, the magnitude of external magnetic field Ho is measured as a change in inductance of the coil when external field Ho is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In FIG. 17 each of bias field B and external field Ho is expressed in the magnitude converted to the coil current.
In this method the position of bias point B changes with factors such as the intensity of the magnetic field generated by the magnet or the positions of the magnet and the core in relation to each other. It is therefore required to maintain the inductance at a specific value when the external magnetic field is zero. However, it is extremely difficult to compensate the instability of the inductance value due to temperature changes and other external perturbations. This method is therefore not suitable for practical applications.
If a rod-shaped magnetic core is used, an open magnetic circuit is provided, so that the effect of hysteresis is generally very small. Assuming that the hysteresis of the core is negligible, the characteristic of variations in inductance is equal when the coil current flows in the positive direction and in the negative direction since the saturation characteristic of the core is independent of the direction of coil current. For example, in FIG. 17 it is assumed that point P+ and point Pxe2x88x92 represent the coil current in the positive direction and the coil current of the negative direction, respectively, whose absolute values are equal to each other. In the neighborhood of these points, the characteristic of variations in inductance with respect to variations in the absolute value of the coil current is equal. Therefore, an alternating current may be applied to the coil such that the core is driven into a saturation region at a peak, and the difference in the amount of decrease in inductance may be measured when positive and negative peak values of the current are obtained. As a result, the difference thus measured is constantly zero when the external magnetic field is zero, which is always the case even when the characteristics of the core change due to temperature changes or external perturbations. In the present patent application a saturation region of the magnetic core means a region where an absolute value of the magnetic field is greater than the absolute value of the magnetic field when the permeability of the core is maximum.
An external magnetic field is assumed to be applied to the core. If external field Ho is applied in the positive direction of the current, as shown in FIG. 17, the inductance value decreases at the positive peak of the current (point Q+ in FIG. 17, for example) and the inductance value increases at the negative peak of the current (point Qxe2x88x92 in FIG. 17, for example). Therefore, the difference between the values is other than zero. Since the difference in inductance depends on the external magnetic field, the external field is obtained by measuring the difference in inductance.
The method thus described is called a large amplitude excitation method in the present patent application, that is, to apply an alternating current to the coil such that the core is driven into a saturation region at a peak, and to measure the difference in the amounts of decrease in inductance when positive and negative peak values of the current are obtained.
Magnetic sensor apparatuses that utilize such a large amplitude excitation method are disclosed in Published Examined Japanese Patent Application Sho 62-55111 (1987), Published Examined Japanese Patent Application Sho 63-52712 (1988), and Published Unexamined Japanese Patent Application Hei 9-61506 (1997), for example. In Published Examined Japanese Utility Model Application Hei 7-23751 (1995), a technique is disclosed to achieve measurement similar to the large amplitude excitation method through the use of two bias magnets.
The large amplitude excitation method is an excellent method since the effects of temperature changes and external perturbations are eliminated. However, it is not so easy to apply an alternating current enough to drive the core into saturation. Accordingly, in prior art the large amplitude excitation method is limited to a magnetic sensor apparatus for detecting a small magnetic field through the use of an amorphous magnetic core and the like having a small saturation field.
For non-contact measurement of a direct current, a method is generally taken to detect a magnetic field generated by a current through the use of a magnetic sensor element. In this method, for example, a magnetic yoke having a gap is provided around a current path, and a magnetic sensor element is placed in the gap. The magnetic field in the gap is measured by the sensor element. Intensity H of the field in the gap is I/g where the current value is I and the gap length is g. Current I is thus obtained by measuring magnetic field H with a magnetic sensor element.
The use of a fluxgate element as a magnetic sensor element will now be considered. A fluxgate element has a feature that the length in the direction in which a magnetic field is applied is relatively long. Therefore, gap length xe2x80x98gxe2x80x99 is relatively long. A shortest length of an actual fluxgate element in the direction in which a magnetic field is applied is about 1 to 5 mm. In addition, a long gap length is acceptable since a fluxgate element has a high sensitivity so that an extremely large magnetic field is not necessary. Accordingly, the gap length of the magnetic yoke of a current sensor apparatus incorporating a fluxgate element is longer than that of a sensor apparatus incorporating any other type of magnetic sensor element such as a Hall element. In an actual design, the gap length of a 100A-level current sensor apparatus is 5 to 10 mm.
This indicates that, if the position of the current path surrounded by the magnetic yoke is close to the gap, the magnetic field inside the gap varies with the magnetic field corresponding to the magnetic flux generated from the current path and not passing through the yoke. Since the magnetic field at a distance of radius xe2x80x98rxe2x80x99 from the current path is I/2xcfx80r, the magnetic field corresponding to the flux not passing through the yoke is greater than the field corresponding to the flux passing through the yoke where r less than g/2xcfx80. As thus described, it is noted that a variation in the position of the current path is one of the greatest factors causing measurement errors for a fluxgate element although such a variation in the current path position will not cause any problem if gap length xe2x80x98gxe2x80x99 is 1 to 2 mm as in a Hall element.
To avoid the above-stated problem, a method may be taken to fix the current path or to use a large magnetic yoke and increase the distance between the current path and the gap. However, the method of fixing the current path sacrifices the convenience of the apparatus in that a current is measured by simply passing an electric wire through the space inside the magnetic yoke. The method of using a large magnetic yoke has a problem that the sensor apparatus is increased in size and weight.
Since not many researches have been made on current sensor apparatuses incorporating a fluxgate element, no prior-art example is found in the method of reducing measurement errors resulting from variations in the position of the current path. However, a current sensor apparatus incorporating a fluxgate element has many features such as reliability better than those of current sensor apparatuses using other magnetic sensor elements. It is therefore very useful in industry to reduce measurement errors resulting from variations in the position of the current path, the only drawback of the appartus using a fluxgate element.
In Published Unexamined Japanese Patent Application Hei 5-99953 (1993), a technique is disclosed for reducing errors in a detected current value caused by an electric wire passing outside a magnetic yoke. However, no consideration is given to errors caused by variations in the position of an electric wire passing through the space inside the yoke. In Published Unexamined Japanese Patent Application Hei 8-15322 (1996), a technique is disclosed for reducing magnetic effects on a magnetic detection element of a magnetic field generated from a conductor to be measured or other external magnetic fields. In this technique a magnetic core is divided into a ring-shaped core on which no feedback winding is placed and an H-shaped core on which a feedback winding is placed. The H-shaped core detects a leakage flux in a gap of the ring-shaped core. The H-shaped core together with the magnetic detection element placed near the H-shaped core is covered with a magnetic shield member. However, this technique has a problem that the configuration is complicated and the current sensor apparatus is large-sized.
The large amplitude excitation method is an excellent method since the effects of temperature changes and external perturbations are eliminated. If the large amplitude excitation method is applied together with the negative feedback method, excellent properties will be expected, according to the principle. Examples in which the negative feedback method is applied to a magnetic sensor apparatus incorporating a fluxgate element are disclosed in Published Unexamined Japanese Patent Application Sho 60-185179 (1985) and Published Unexamined Japanese Patent Application Hei 9-257835 (1997).
However, a fluxgate current sensor apparatus incorporating a fluxgate element involves the following drawback resulting from the measurement principle thereof. Since the fluxgate current sensor apparatus is made up of a sampling system, the measurement frequency band is limited. In other words, with regard to the fluxgate current sensor apparatus, the response frequency band, that is, the frequency band that responds to variations in a current to be measured, is not allowed to exceed the excitation frequency which is the frequency of the excitation current, that is, an alternating current applied to the coil of the fluxgate element, due to the Nyquist frequency which is the threshold frequency of response. With regard to the fluxgate current sensor apparatus, since sampling is made at two points of positive and negative in one cycle of the excitation current, the sampling frequency is twice the excitation frequency. The Nyquist frequency is half the sampling frequency.
Moreover, when the negative feedback method is applied to the fluxgate current sensor apparatus, it is not so easy to widen the band of the negative feedback system having a sufficiently large loop gain since it is required to give enough consideration to measures against oscillation and so on.
As thus described, the prior-art fluxgate current sensor apparatus has the problem that although the response frequency band is wide enough for ordinary applications, it is difficult to further widen the response frequency band.
As disclosed in Published Unexamined Japanese Patent Application Hei 1-265168 (1989) and Published Unexamined Japanese Patent Application Hei 4-93772 (1992), for example, a method is known for widening the response frequency band of a current sensor apparatus in general, which is not limited to a fluxgate current sensor apparatus. In this method, a high frequency component of a current to be measured is detected by a coil coupled through alternating-current and magnetic coupling to a current path through which the current to be measured passes. On the other hand, a low frequency component of the current including a direct current is detected by a magnetic sensor element. Those two detection signals are combined. This method is called an alternating-current (AC) coupling method in the present patent application.
However, in the prior-art current sensor apparatus utilizing the AC coupling method, as disclosed in the above-mentioned publications, the coil for detecting a high frequency component is provided in the magnetic yoke itself. If the negative feedback method is applied, the coil for detecting a high frequency component functions as a coil for generating a feedback magnetic field, too, in many cases. As a result, the prior-art current sensor apparatus utilizing the AC coupling method has a problem that manufacturing costs of the apparatus are raised since winding the coil around the magnetic yoke requires difficult techniques, and the large number of turns of the coil makes the apparatus large-sized.
Furthermore, the prior-art current sensor apparatus utilizing the AC coupling method has the following essential problem. The prior-art techniques utilizing the AC coupling method have been mainly developed for a current sensor apparatus incorporating a Hall element as the magnetic sensor element. In such an apparatus, it is acceptable that the gap of the magnetic yoke is small. In contrast, in the current sensor apparatus incorporating a fluxgate element as the magnetic sensor element, the gap of the yoke is greater than that of the apparatus incorporating a Hall element. As a result, the current sensor apparatus incorporating a fluxgate element has a problem that the coil for detecting a high frequency component is large-sized and the apparatus is expensive, due to the following reason.
Inductance L of the coil for detecting a high frequency component wound around the magnetic yoke is expressed as L=Kxc2x7N2 where the AL value (the inductance value per one turn of the coil) of the magnetic yoke is K and the number of turns of the coil is N.
It is assumed that the magnetic yoke is made of a high-permeability ferrite material having a cross-sectional area of about 5 by 5 mm. If the length of the gap of the yoke is 1 mm, that is, a typical length for a current sensor apparatus incorporating a Hall element, the AL value K of the yoke is about 1 xcexcH/T. If the length of the gap of the yoke is 10 mm, that is, a typical length for a current sensor apparatus incorporating a fluxgate element, the AL value K of the yoke is about 0.1 xcexcH/T, which is {fraction (1/10)} of the value K obtained when the Hall element is used.
If the direct current resistance of the coil for detecting a high frequency component is xe2x80x98rxe2x80x99, cutoff frequency xe2x80x98fxe2x80x99 of the coil is f=r/2xcfx80Kxc2x7N2. Therefore, in order to obtain the same cutoff frequency when the fluxgate element is used, it is required that, for example, the number of turns of the coil is about three times that of the apparatus using the Hall element and the coil is made of a wire having a cross section about three times as large as that of the coil of the apparatus using the Hall element.
As thus described, if the prior-art AC coupling method is directly applied to the current sensor apparatus incorporating the fluxgate element, the coil for detecting a high frequency component is large-sized and the apparatus is expensive.
In addition, in the prior-art techniques using the AC coupling method, no consideration is given to the cutoff frequency and the excitation frequency of the coil for detecting a high frequency component that are problems specific to the sampling system. Therefore, if the prior-art AC coupling method is directly applied to the current sensor apparatus incorporating the fluxgate element, a beat may be produced from the frequency of a varying component of a current to be measured and the excitation frequency.
It is a first object of the invention to provide a current sensor apparatus for reducing measurement errors resulting from variations in the position of a current path passing through the space inside a magnetic yoke, without losing the convenience of use and without increasing the current sensor apparatus in size and weight.
It is a second object of the invention to provide a current sensor apparatus for widening the response frequency band while suppressing a beat generated by the frequency of a varying component of a current to be measured and the excitation frequency, and reducing difficulties in manufacturing the apparatus and the size of the apparatus.
A first current sensor apparatus of the invention comprises: a ring-shaped magnetic yoke through which a magnetic flux generated by an electric current flowing through an electric path passes, the yoke surrounding the current path, part of the yoke having a gap; and a magnetic sensor element placed in the gap of the magnetic yoke and provided for detecting a magnetic field in the gap generated by the current flowing through the current path. A magnetic path of the flux passing through the magnetic yoke includes: a first magnetic path, mainly passing through the magnetic sensor element, through which a part of the flux passing through the magnetic yoke passes; and a second magnetic path through which another part of the flux passing through the magnetic yoke passes.
According to the first current sensor apparatus of the invention, the magnetic sensor element detects the magnetic field in the gap generated by the current flowing through the current path, based on the magnetic flux passing through the first magnetic path. The second magnetic path may be used for a function other than detecting a magnetic field by the magnetic sensor element.
The first current sensor apparatus may comprise a magnetic field interrupter, placed between the current path and the gap, for interrupting a magnetic field corresponding to a magnetic flux that is generated by the current passing through the current path and does not pass through the magnetic yoke, so that the magnetic field is cut off from the magnetic sensor element. In addition, the second magnetic path may be made up of the field interrupter. In this current sensor apparatus the field interrupter cuts off the magnetic field corresponding to the magnetic flux that is generated by the current passing through the current path and does not pass through the magnetic yoke from the magnetic sensor element.
In the first current sensor apparatus comprising the field interrupter, the field interrupter may be separated from the magnetic yoke or may be integrated with the magnetic yoke. Part of the field interrupter may have a gap. A center position of the gap of the field interrupter may be off a straight line drawn through a center of the current path and a center of the gap of the magnetic yoke. The field interrupter may be made of a magnetic substance. The magnetic sensor element may be a fluxgate magnetic sensor element, for example.
The first current sensor apparatus may further comprise a high frequency component detection coil for detecting a high frequency component of the current flowing through the current path, and the detection coil may be placed in the second magnetic path.
In this current sensor apparatus the magnetic sensor element detects the magnetic field in the gap generated by the current flowing through the current path. The detection coil detects a high frequency component of the current flowing through the current path. In the apparatus the magnetic sensor element and the detection coil are placed in the different magnetic paths. Therefore, it is possible to determine the cutoff frequency and so on for each of the magnetic sensor element and the detection coil independently.
The first current sensor apparatus comprising the high frequency component detection coil may further comprise an attenuation means, placed in the first magnetic path, for attenuating a frequency component of the magnetic flux passing through the first magnetic path that is higher than a specific cutoff frequency.
In the first current sensor apparatus comprising the high frequency component detection coil, the magnetic sensor element may have a magnetic core placed in the first magnetic path and a sensor coil wound around the core and provided for detecting a magnetic field corresponding to the flux passing through the first magnetic path. In this case the apparatus may further comprise: a drive means for driving the sensor coil by supplying an alternating excitation current that drives the core into a saturation region to the sensor coil; and a measurement means for measuring the current passing through the current path by detecting variations in inductance of the sensor coil. In the present invention the magnetic core is a core made of a magnetic substance having a magnetic saturation property on which the coil is wound. To drive the sensor coil means to supply an alternating current to the sensor coil.
The current sensor apparatus comprising the drive means and the measurement means may further comprise an attenuation means, placed in the first magnetic path, for attenuating a frequency component of the magnetic flux passing through the first magnetic path that is higher than a specific cutoff frequency. The cutoff frequency may be equal to or lower than a Nyquist frequency obtained from a frequency of the excitation current. In the current sensor apparatus comprising the drive means and the measurement means, the drive means may have a series resonant circuit part of which is made up of the sensor coil and may supply a resonant current flowing through the series resonant circuit as the excitation current to the sensor coil. The current sensor apparatus comprising the drive means and the measurement means may further comprise a current supply means for supplying an electric current to the sensor coil, the current including a direct current and having a frequency different from a frequency of the excitation current. In this case the current supply means may supply a negative feedback current to the sensor coil for negative feedback of an output of the measurement means to the sensor coil. The high frequency component detection coil may form a path for supplying the negative feedback current to the sensor coil.
In the current sensor apparatus comprising the drive means and the measurement means, the measurement means may have: an inductance element connected to the sensor coil in series; and a differentiation circuit for differentiating a voltage generated across the inductance element and outputting a signal responsive to the current flowing through the current path.
In the first current sensor apparatus comprising the high frequency component detection coil, the second magnetic path may include a gap.
In the first current sensor apparatus comprising the high frequency component detection coil, the second magnetic path may be located between the current path and the first magnetic path. In this case, the current sensor apparatus may further comprise a magnetic field interrupting member made of a magnetic substance and placed in the second magnetic path, the member interrupting a magnetic field corresponding to a magnetic flux that is generated by the current passing through the current path and does not pass through the magnetic yoke, so that the magnetic field is cut off from the magnetic sensor element. In addition, the high frequency component detection coil may be wound around the field interrupting member.
A second current sensor apparatus of the invention comprises: a ring-shaped magnetic yoke through which a magnetic flux generated by an electric current flowing through an electric path passes, the yoke surrounding the current path, part of the yoke having a gap; a magnetic sensor element placed in the gap of the magnetic yoke and provided for detecting a magnetic field in the gap generated by the current flowing through the current path; and a magnetic field interrupter, placed between the current path and the gap, for interrupting a magnetic field corresponding to a magnetic flux that is generated by the current passing through the current path and does not pass through the magnetic yoke, so that the magnetic field is cut off from the magnetic sensor element.
In the second current sensor apparatus the field interrupter cuts off the magnetic field corresponding to the magnetic flux that is generated by the current passing through the current path and does not pass through the magnetic yoke from the magnetic sensor element.
In the second current sensor apparatus, the field interrupter may be separated from the magnetic yoke or may be integrated with the magnetic yoke. Part of the field interrupter may have a gap. A center position of the gap of the field interrupter may be off a straight line drawn through a center of the current path and a center of the gap of the magnetic yoke. The field interrupter may be made of a magnetic substance. The magnetic sensor element may be a fluxgate magnetic sensor element, for example.
Other and further objects, features and advantages of the invention will appear more fully from the following description.